Tri-signal electromagnetic article surveillance system

ABSTRACT

In a method for electronic surveillance of articles in a given area, one preselects first and second zones in the area respectively for surveillance and non-surveillance and affixes to the articles devices adapted to provide a first output indicative of an alarm condition upon receipt of first predetermined signals and further adapted to provide a second output not indicative of an alarm condition upon receipt of such first signals together with second predetermined signals diverse from the first signals. One transmits the first signals in the area, and transmits the second signals at least into the second zone, such that devices on articles present in the second zone provide the second output. The first signals may be microwave and modulated low-frequency signals. The second signals may be unmodulated low-frequency signals. Systems and antenna configuration are also shown.

FIELD OF THE INVENTION

The present invention relates generally to article pilferage control andpertains more particularly to an enhanced method and system for suchusage.

BACKGROUND OF THE INVENTION

The electronic surveillance art has seen continued advance in the pastdecade in both sensitivity and consistency of detection of suitablytagged articles in unauthorized transport thereof from secured areas.

One significant advance is disclosed in commonly assigned U.S. Pat. No.3,895,368 issued on July 15, 1975 to Gordon et al., wherein joint highand modulated low frequency fields, respectively radiant electromagneticand electrostatic, are established in the surveillance or control zoneof an area also including an article display zone. Articles of concernhave receptor-reradiator devices secured thereto and such devices havecapability for transmitting an output signal having preselectedcorrespondence to energy incident thereon.

In the Gordon et al. systems, the high-frequency field results from acontinuous wave (CW) microwave frequency transmitter, e.g., operating atnine hundred and fifteen megahertz, and the low frequency field resultsfrom a transmitter operating at a substantially lower frequency with atime-based, frequency-modulation (f-m) characteristic. An alarm or otherindication of unauthorized article presence in the surveillance zone isprovided when a receiver receives a receptor-reradiator output signaltransmission which includes content corresponding to the high and lowfrequencies and detects the modulation characteristic of the latter.Output alarm indication is preferably delayed in the '368 patent untilthere is a given repetitive occurrence of receipt of the proper detectedalarm signal.

Alternative effective alarm condition sensing is achieved in anothercommonly assigned patent, U.S. Pat. No. 4,139,844 issued to Reeder onFeb. 13, 1979, wherein different modulated low-frequency signals arepropogated with the microwave energy in the surveillance zone. Here, analarm condition occurs only where a signal is detected which has contentcorresponding to a composite characteristic of the plural low frequencysignals.

Fundamental to these systems of the assignee hereof is a recognition ofneed for a high-frequency carrier, the microwave transmission, and ofthe inherent difficulties of limiting the transmission pattern thereof,and of the detectable alarm intelligence therewith, through modulation,of a low-frequency carrier whose propagation pattern is comparativelycontrollable. Control of the pattern of the low frequency field is thuspractical and gives rise to some measure of distinction as between thesurveillance zone and an adjacent article display zone in which taggedarticles are to be present without creating an alarm condition.

A phenomenon of wave propagation is that pattern control is inexact anda so-called "over-range" characteristic is inherent, i.e., patterns willextend beyond anticipated or planned ranges, dependent upon variousfactors, such as the presence of pattern-influencing extraneous objects.Mechanical shielding, e.g., conductive screens, have provided a degreeof limitation on over-range, but are generally undesirable, based onphysical size, aesthetic requirements and the impracticality ofphysically shielding large areas. In the absence of such screening,tagged articles outside of the surveillance zone can give rise to alarmindication and full usage of an article display area may accordingly bediminished.

One known approach to overrange control is shown in U.S. Pat. No.4,212,002, issued to Williamson on July 8, 1980. In the system of the'002 patent, a high-frequency field is established in an area and afirst low-frequency field with a modulation characteristic isestablished in a surveillance zone. A second low-frequency field with amodulation characteristic is established adjacent the boundary of thesurveillance zone by antenna means transmitting energy directionallyaway from the surveillance zone. The frequency of the second fieldsignal is distinct from that of the first field, such that afrequency-modulation receiver at the surveillance zone can discriminatebetween receptor-reradiator output transmissions from articles in thesurveillance zone and in the boundary zone. The operative mechanism isthe f-m receiver's "locking on" to the strongest of the differentfrequency carriers within its pass band. Transmitters of articles in theadjacent boundary zone will provide output signals having the secondfield carrier dominating over the first field carrier, and accordinglywill not cause alarm indication. Upon receiving such transmissions fromthe boundary zone, the receiver operates in its customary mode, i.e.,detecting the modulation characteristic of the second frequency carrier.Output may accordingly be provided, advising of the approach of anarticle toward the surveillance zone. In applicants' view, the operativemechanism of the '002 system, namely, the need to lock on to pluralcarrier frequencies, can give rise to a lessening of sensitivity of thereceiver to transmissions issuing in the surveillance zone.

SUMMARY OF THE INVENTION

The present invention has as its primary object the provision ofimproved methods and systems for electronic article surveillance.

A particular object of the invention is to permit enhanced control ofoverranging in article surveillance systems.

In attaining these and other objects, the invention introduces in thetype of system discussed above in connection with the Gordon et al '368patent active circuit means for electronically shielding a zone (articledisplay zone) preselected for tagged article presence without attendantgeneration of alarm conditions. By this measure, although such articledisplay zone may have transmission levels therein, of both the microwaveenergy and the modulated low-frequency energy of magnitude which would,in the absence of the electronic shielding, cause alarm indication, thedetector station receiver, in accordance with the invention herein, isrendered selectively insensitive to the same and an alarm condition isnot generated.

In broad aspect, the invention provides system and method for electronicsurveillance of articles in a given area, comprising the steps ofpreselecting first and second zones in said area respectively forsurveillance and non-surveillance, affixing to the articles devicesadapted to provide a first output indicative of an alarm condition uponreceipt of first predetermined signals and further adapted to provide asecond output not indicative of an alarm condition upon receipt of suchfirst signals together with second predetermined signals diverse fromthe first signals, transmitting the first signals in the area, andtransmitting the second signals into adjacent portions of the first andsecond zones, such that said devices on articles present in suchadjacent portions provide the second output. The first signals may bethe microwave and modulated low-frequency signals of the '368 patent.The second signals may be unmodulated low-frequency signals.

In a preferred practice in accordance with the invention, for electronicarticle surveillance in an area having a surveillance zone and anarticle display zone adjacent said surveillance zone, a method includesthe steps of: transmitting into the area a high-frequency signalcomprised of an unmodulated carrier and a primary low-frequency signalcomprised of a carrier with a modulation characteristic; and affixing toarticles in the area receptor-reradiator devices adapted fortransmitting output signals having content corresponding to transmittedsignals incident thereon. Receiver-detector means are disposed in thesurveillance zone for receiving the receptor-reradiator means outputsignals, the receiver-detector means having capability for providingoutput indication of the modulation characteristic therein if same is inpredetermined ratio to low-frequency carrier content thereof, i.e., isabove a given threshold level assigned for detection purposes, andthereupon generating an alarm condition. The method further involvestransmitting into the area an auxiliary low-frequency signal comprisedof an unmodulated carrier and thereby selectively increasinglow-frequency carrier content of the receptor-reradiator means outputsignals for receptor-reradiator means in the article display zone tothereby cause the ratio of the modulation characteristic in suchselected receptor-reradiator means output signals to low-frequencycarrier content thereof to be less than such predetermined ratio andthereby precluding generation of the alarm condition.

The mechanism underlying the practice of the invention will be seen notto involve a frequency hunting or "lock on" shifting as in theWilliamson patent above discussed. Rather, the invention looks, in thereceiver-detector to the quantum of total low-frequency carrier presentand the modulation characteristic is not detectable where large carriercontent is present, as contributed to by the auxiliary field. As anarticle is carried into the surveillance zone, however, lessenedauxiliary field carrier content is present in the receiver andmodulation characteristic detection occurs.

The frequency of the auxiliary low-frequency carrier is desirably equalto that of the primary low-frequency carrier. The modulationcharacteristic of the primary low-frequency signal is selected to be atime-permutative characteristic. In one preferred practice discussed indetail below, the primary low-frequency transmission is a one hundredkilohertz carrier which is frequency-shift-keyed at eight hundred hertz.The auxiliary low-frequency transmission is an unmodulated one hundredkilohertz carrier.

The method is further desirably practiced through the use of first,second and third antennas for transmission respectively of thehigh-frequency signal, the primary low-frequency signal and theauxiliary low-frequency signal, and the third antenna is locatedgenerally at a line of demarcation between the surveillance zone and thearticle display zone and may be made visible to provide indication ofsuch demarcation.

Systems of the invention are configured for practice of the describedmethod, as below shown and discussed.

The foregoing and other objects and features of the invention will befurther understood from the following detailed description of preferredembodiments thereof and from the drawings wherein like referencenumerals identify like parts throughout.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a system in accordance with the invention.

FIG. 2 is a schematic diagram illustrative of zone definition in asurveillance area without the auxiliary field transmitter.

FIG. 3 is a schematic diagram illustrative of zone definition in asurveillance area and placement of radiators therein for implementingthe FIG. 1 system.

FIG. 4 depicts signal waveforms at various stages of the FIG. 1 systemin part under operating conditions with and without the auxiliary fieldtransmitter.

FIG. 5 is a side sectional view of the auxiliary field antenna shown intypical placement and with indication of the field generated thereby.

DESCRIPTION OF PREFERRED EMBODIMENTS AND PRACTICES

Referring to FIG. 1, system 10 includes components akin to those of theGordon et al. '368 patent to which incorporating reference is herebymade. High-frequency UHF generator or transmitter 12 may operate at ninehundred and fifteen or twenty four hundred and fifty megahertz, whichare known microwave frequencies for use in surveillance practices. Theoutput of high-frequency generator 12 is applied over line 14 to a 3 dbisolator pad 16. A bandpass filter 18 is connected to pad 16 by line 20and feeds splitter 22 over line 24. Filter 18 has a center frequency ofnine hundred and fifteen megahertz where system UHF transmission is atthe first above-noted frequency. Splitter 22 has output lines 26 and 28connected to UHF antenna system 30. System 30 is depicted in the '368patent as including individual transmitting antennas for connectionrespectively to lines 26 and 28, the antennas being mounted on oppositesides of the area to be controlled in corresponding enclosures orpedestals, thereby establishing an electromagnetic field of microwaveenergy in the controlled space therebetween.

A second pair of antennas are in UHF antenna system 30 and are mountedacross the controlled space from the corresponding transmitting antennasfor receipt of signals existing in the controlled space and forfurnishing same respectively on output lines 32 and 34 to combiner 36,whose output is fed over line 38 to bandpass filter 40. The output offilter 40 is applied by line 42 to one input of balanced mixer 44. Asecond mixer 44 input is furnished with a signal at nine hundred andfifteen megahertz from a low power level output of generator 12 overline 46. The output of mixer 44 is conveyed by line 48 tofrequency-modulation detector 50.

Turning to the upper portion of FIG. 1, a voltage-controlled pulsegenerator 52 applies output pulses on line 54 having a frequency ofeight hundred hertz for frequency-shifting pulse generator 56. Generator56 has a center frequency of one hundred kilohertz and, at the rate ofthe line 54 pulses, deviates by plus and minus one and three-tenthskilohertz. The output of generator 56 is fed by line 58 to low-frequencyantenna system 60, also shown in the '368 patent to have a resonant-typelow pass filter connected to line 58, whereby the square wave input isconverted to a sinusoidal signal of like frequency. The filter output ispower amplified and applied through step-up transformers to respectivefoil antennas. The foils are located in the above-noted pedestals onopposed sides of the area being controlled and establish anelectrostatic field to ground.

Line 62 applies the output of generator 52 to delay multivibrator 64,the output of which is fed by line 66 to the input of a reference pulsemultivibrator 68. The output of multivibrator 68 is furnished with thatof detector 50 to alarm system 70 over lines 72 and 74.

Alarm system 70 may be of type shown in the '368 patent, i.e., havinglogic circuitry to compare the inputs received on lines 72 and 74 andnoting correspondence therebetween in a repetitive pattern and thereuponproviding output alarm indication.

Completing the circuit elements of FIG. 1, the system of the inventionincludes auxiliary transmission driver 76, which generates sinusoidaloutput at one hundred kilohertz, without modulation. Driver 76 furnishesits output over line 78 to auxiliary antenna system 80 which, asdiscussed below, may comprise an antenna in the form of an extrusionplaced selectively in relation to the above discussed antennas.

Referring to FIG. 2, In typical electronic surveillance planning, aretail area 84 may have a surveillance zone 86 selected to extend fromstorefront 88 to depth 90 and an article display zone 92 extendingrearwardly from the exit zone in depth 94. Clear definition of a line 96of demarcation between the zones is evidently desirable. Thus, theretailer desires to maximize article display area while, at the sametime, needs practical assurance that tagged articles may be carriedfreely throughout such zone 92 without activation of an alarm.

In presently commercialized systems, the energy radiation pattern ofhigh and low frequency levels which causes receptor-reradiatortransmissions, is not precisely defined and is typically ofconfiguration indicated at 98 in the FIG. 2 plan view of retail area 84.Cross-hatched area 100 thus represents an active area within such fieldrearward of desired demarcation line 96 and sideward of lines 100a and100b.

Tagged articles carried into area 100 are thus within the alarmcondition zone and area 100 is manifestly unusable through for articledisplay purposes.

Turning to FIG. 3, auxiliary low frequency antenna 102 of the FIG. 1auxiliary antenna system is shown as having a rectilinearly extendingcentral section 104 and end sections 106 and 108 extending from centralsection 104 slightly outwardly of pedestals 110 and 112. Antenna 102 ispositioned with its central section in alignment with zone demarcationline 96. By system setup below discussed, it is found that the systemfield of sensitivity can now be conformed to that indicated at 111 inFIG. 3, i.e., ending rearwardly at demarcation line 96 and ofconfiguration corresponding to the physical and hence observable outlineof antenna 102. In placement, antenna 102 may be suspended from theceiling of retail area 84 or may be otherwise elevated with respect tothe antennas of the high and modulated low frequency field antennasystems. As such, antenna 102 provides a practical visual embodiment ofdemarcation line 96 and a retailer is given knowledge of the extent ofthe system field of sensitivity, and may accordingly both maximizearticle display usage in zone 92 adjacent the demarcation line and enjoythe comfort of minimum, if any, alarm conditions occurring in displayarea 92.

In system setup practice, one first delineates surveillance zone 86 andsystem-insensitive, article display zone 92, identifying thusly thedesired location of demarcation line 96. Antenna 102 is now located insubstantial registry with line 96. The system, except for auxiliarytransmission driver 76 of FIG. 1, is energized, thereby effectingtemporarily a field of distribution 98 of FIG. 2. A tag is now disposedin cross-hatched area 100 rearwardly of line 86. The leftward signals ofFIG. 4 will now occur. Thus, the top signal trace, line 58 of FIG. 1,will be pulsed HI (+d. c. voltage) from t1 to t2 and from t3 to t4, andwill be LO (d. c. ground potential) from t2 to t3. Antenna system 60will furnish a one hundred and one and three-tenths (101.3) kilohertzburst between t1 and t2 and between t3 and t4 and will furnish aninety-eight and seven-tenths (98.7) kilohertz burst during thealternate time periods, as indicated. Line 66 of FIG. 4 shows the pulseoutput provided by delay multivibrator 64 of FIG. 1. It will be seenthat the leading edge of the pulse coincides with the leading edge ofthe positive going pulse output of pulse generator 52 on line 54. Thetrailing edge of the pulse provided by multivibrator 64 may beadjustable by appropriate means not shown. The delay produced bymultivibrator 64 is thereby adjusted to be equal to the normal delayencountered by the signals in passing through the equipment both intothe electrostatic (low-frequency) field and back on the modulatedcarrier through the balanced mixing and detecting circuitry.

The trailing edge of the pulse on line 66 is employed to trigger thereference pulse multivibrator 68 of FIG. 1, whose output is shown online 72 in FIG. 4. The width of the pulse produced by this multivibratormay be adjustable by means not shown so that such pulse width coincideswith the pulse width detected from an actual tag in the controlledspace.

The receptor-reradiator of the tag above placed in cross-hatched area100 of FIG. 2 may comprise the commercially known combined dipoleantenna with diode. It will develop a modulated output in correspondencewith energy incident thereof, inclusive of the modulation characteristicof the low-frequency transmission, the low-frequency carrier and themicrowave transmission. On receipt by the FIG. 1 system, such signal isapplied to balanced mixer 44. In known manner, mixer 44 will remove themicrowave frequency carrier component and supply the 101.3 and 98.7kilocycle components as detected thereby over line 48 to f-m detector 50for conversion to a square wave pulse having the form shown at line 74in FIG. 4. An alarm condition will be seen to be present in FIG. 4,since the signals on lines 72 and 74 are in correspondence.

During the time period t4-t5 of FIG. 4, in the setup example at hand,auxiliary transmission driver 76 of FIG. 1 is energized at any givenoutput power level, applying to line 78 a continuous wave at unmodulatedone hundred kilocycles. Assuming such power level to be below thatrequired for alarm suppression, energization of driver 76 has as yet noconsequence, the event on line 74 during t1-t3 and t3-t4 recurringduring t5-t6.

At t6, the output of driver 76 is increased to such higher level as willbring on the event at t7, i.e., wherein, despite continous microwavetransmission and a concurrent burst on line 58, line 74 does not stepfrom LO to HI in cadence with line 72. At the receiver, sufficientenergy of unmodulated low-frequency carrier has been transmitted by thetag to the receiver as to decrease its sensitivity to the alarmcondition signal present but dwarfed in the receiver input. The ratio ofmodulation characteristic in the received signal to carrier energy isbelow that ratio which is necessary to attain detection threshold of thereceiver. As, however, such system setup article, and any subsequentlytagged article is transported increasingly forwardly of demarcation line96, same receives and reradiates substantially less auxiliary(unmodulated) low-frequency energy, whereupon the ratio of modulationcharacteristic to low-frequency carrier in the receiver increases tocause alarm condition occurrence.

Turning to FIG. 5, the foregoing is seen graphically, Antenna 102 isshown in section and comprises a metal pipe, obtained preferably byextrusion. Field lines 114 and 116 issue radially of the antenna andbend to ground in respective adjacent portions of surveillance orcontrol zone 86 and display or uncontrolled zone 92. To the extent thatsome modulated low-frequency may issue from zone 86 into zone 92, thesame is dominated, within such adjacent surveillance zone portions, bythe antenna 102 output. However, as the article with tag is moved to theoutskirts of the illustrated field lines, i.e., leftwardly into thedepth of the controlled zone, unmodulated carrier is decreasinglyreceived and the modulated carrier predominates.

Various changes to the particularly described embodiment and variationsin the discussed methods may be introduced without departing from theinvention. By way of example, the auxiliary antenna 102 may standupright with and adjacent the pedestals containing the other antennas,or may be disposed on a floor, with like effects as above discussed.Also, it will be appreciated that the invention may be introduced in orpracticed with electronic surveillance systems other than the abovediscussed system of the '368 patent. By way of example, such system mayemploy single antennas in place of the antenna pairs above discussed, inwhich case each such antenna is employed alternately in a transmit and areceive mode. Likewise, alarm system 70 above is typical and may berepaced by other signal processing practices. Accordingly, the preferredembodiments and methods above are intended in an illustrative and not ina limiting sense. The true spirit and scope of the invention is setforth in the following claims.

We claim:
 1. A system for electronic article surveillance in an areahaving a surveillance zone and an article display zone adjacent saidsurveillance zone, said system including:(a) means for transmitting intosaid area a high-frequency signal; (b) means for transmitting into saidarea a primary low-frequency signal comprised of a carrier with amodulation characteristic; (c) receptor-reradiator means affixed toarticles in said area for transmitting output signals having contentcorresponding to transmitted signals incident thereon and inclusive ofsaid modulation characteristic and such low-frequency carrier; (d)receiver-detector means in said surveillance zone for receiving suchreceptor-reradiator means transmitted signals and having capability forproviding output indication of said modulation characteristic therein ifsame is in predetermined ratio to low-frequency carrier content thereofand thereupon to generate an alarm condition; and (e) means fortransmitting into said area an auxiliary low-frequency signal comprisedof an unmodulated carrier for selectively increasing low-frequencycarrier content of said receptor-reradiator means transmitted signalsfor receptor-reradiator means in said article display zone to cause theratio of said modulation characteristic in such selectedreceptor-reradiator means transmitted signals to low-frequency carriercontent thereof to be less than said predetermined ratio and thereuponto preclude generation of said alarm condition.
 2. The system claimed inclaim 1 wherein said means (e) transmits said auxiliary signal at thefrequency of said means (b) carrier.
 3. The system claimed in claim 2wherein said modulation characteristic of said means (b) signal is atime-permutative characteristic.
 4. The system claimed in claim 3wherein said modulation characteristic of said means (b) signal is afrequency-modulated characteristic.
 5. The system claimed in claim 1wherein said means (a), (b) and (e) include respective first, second andthird antenna means for transmission respectively of said high-frequencysignal, said primary low-frequency signal and said auxiliarylow-frequency signal, said third antenna means being located generallyat a line of demarcation between said surveillance zone and said articledisplay zone.
 6. The system claimed in claim 5 wherein said thirdantenna means is located at higher elevation than said first and secondantenna means.
 7. The system claimed in claim 5 wherein said thirdantenna means is visibly discernible, thereby providing visibility ofsaid demarcation line.
 8. The system claimed in claim 5 wherein at leastsaid second antenna means are a plurality of second antennas and whereinsaid third antenna means is of a configuration having a central expansegenerally in facing relationship to said second antennas and endsections outwardly of of said second antennas.
 9. A method forelectronic article surveillance in an area having a surveillance zoneand an article display zone adjacent said surveillance zone, said methodincluding the steps of:(a) transmitting into said area a high-frequencysignal; (b) transmitting into said area a primary low-frequency signalcomprised of a carrier with a modulation characteristic; (c) affixing toarticles in said area receptor-reradiator devices adapted fortransmitting output signals having content corresponding to transmittedsignals incident thereon and inclusive of said modulation characteristicand such low-frequency carrier; (d) disposing receiver-detector means insaid surveillance zone for receiving said receptor-reradiator meansoutput signals and having capability for providing output indication ofsaid modulation characteristic therein if same is in predetermined ratioto low-frequency carrier content thereof and thereupon generating analarm condition; and (e) transmitting into said area an auxiliarylow-frequency signal comprised of an unmodulated carrier and therebyselectively increasing low-frequency carrier content of saidreceptor-reradiator means output signals for receptor-reradiator meansin said article display zone to thereby cause the ratio of saidmodulation characteristic in such selected receptor-reradiator meansoutput signals to low-frequency carrier content thereof to be less thansaid predetermined ratio and thereby precluding generation of said alarmcondition.
 10. The method claimed in claim 9 wherein the frequency ofsaid auxiliary low-frequency signal is selected to be the same as thefrequency of said primary low-frequency signal.
 11. Themethod claimed inclaim 9 wherein said modulation characteristic of said primarylow-frequency signal is selected to be a time-permutativecharacteristic.
 12. The method claimed in claim 11 wherein saidmodulation characteristic is selected to be a frequency-modulatedcharacteristic.
 13. The method claimed in claim 9 wherein said steps(a), (b) and (e) are practiced through the use of first, second andthird antenna means for transmission respectively of said high-frequencysignal, said primary low-frequency signal and said auxiliarylow-frequency signal, and wherein said third antenna means is locatedgenerally at a line of demarcation between said surveillance zone andsaid article display zone.
 14. The method claimed in claim 13 whereinsaid third antenna means is located at higher elevation than said firstand second antenna means.
 15. The method claimed in claim 13 whereinsaid third antenna means is rendered visibly discernible, therebyproviding visibility of said demarcation line.
 16. The method claimed inclaim 13 wherein at least said second antenna means are a plurality ofsecond antennas and wherein said third antenna means is selected to beof a configuration having a central expanse generally in facingrelationship to said second antennas and end sectiona outwardly of saidsecond antennas.
 17. A method for electronic surveillance of articles ina given area, comprising the steps of:(a) preselecting first and secondadjacent zones in said area respectively for surveillance and articledisplay; (b) affixing to said articles devices adapted to provide outputtransmission selectively upon receipt thereby of predetermined signals,said output transmission having signal content related to saidpredetermined signals; (c) transmitting into said area first and secondof said predetermined signals, respectively of high-frequency carrierand of low-frequency carrier with a modulating characteristic andtransmitting a third of said predetermined signals of unmodulatedlow-frequency carrier and thereby distinct from both said first andsecond signals; and (d) generating an alarm indication selectively whensaid output transmissions of such article-affixed devices have signalcontent related to said first and second predetermined signals andcontent of said third predetermined signal in amplitude less than apre-established amplitude level of said third predetermined signal. 18.The method claimed in claim 17 wherein said step (c) is practiced bytransmitting said third signal in carrier frequency equal to said secondfrequency carrier.
 19. The method claimed in claim 17 wherein said step(c) is practiced by effecting said transmissions of said first, secondand third signals through the use of respective first, second and thirdantenna means, said third antenna means being placed at the location ofsuch adjacency of said first and second adjacent zones.
 20. The methodclaimed in claim 19 wherein said third antenna means is located at ahigher elevation than said second antenna means.
 21. The method claimedin claim 19 wherein said second antenna means comprises plural secondantennas and wherein said step (c) is practiced by disposing a centralsection of said third antenna means at said demarcation line in facingrelationship to said plural second antennas and providing said thirdantenna means with end sections outwardly of said plural secondantennas.
 22. A method for electronic surveillance of articles in agiven area, comprising the steps of:(a) preselecting first and secondzones in said area respectively for surveillance and non-surveillance;(b) affixing to said articles devices adapted to provide a first outputindicative of an alarm condition upon receipt of first predeterminedmodulated signals and further adapted to provide a second output notindicative of an alarm condition upon receipt of such first signalstogether with second predetermined unmodulated signals of amplitudegreater than a preselected amplitude level; (c) transmitting said firstsignals in said area; and (d) transmitting said second signalsselectively into adjacent portions of said first and second zones, suchthat said devices on articles present in said adjacent portions providesaid second output.
 23. The method claimed in claim 22 wherein saidfirst signals are selected to comprise a high-frequency signal and amodulated low-frequency signal and said second signals are selected tocomprise an unmodulated low-frequency signal.
 24. The method claimed inclaim 22 wherein said second signals are transmitted by use of anantenna located generally at a line of demarcation between said firstand second zones.
 25. The method claimed in claim 24 wherein saidantenna is configured to transmit said second signals substantially inthe same pattern in each of said adjacent portions of said first andsecond zones.
 26. A system for electronic surveillance of articles in anarea having respective controlled and uncontrolled zones, said systemcomprising:(a) devices affixed to said articles and adapted to provide afirst output indicative of an alarm condition upon receipt of firstpredetermined modulated signals and further adapted to provide a secondoutput not indicative of an alarm condition upon receipt of such firstsignals together with second predetermined unmodulated signals ofamplitude greater than a preselected amplitude level; (b) means fortransmitting said first signals in said area; and (c) means fortransmitting said second signals selectively into adjacent portions ofsaid first and second zones, such that said devices on articles presentin said adjacent portions provide said second output.
 27. The systemclaimed in claim 26 wherein said first signals comprise a high-frequencysignal and a modulated low-frequency signal and said second signalscomprise an unmodulated low-frequency signal.
 28. The method claimed inclaim 26 wherein said means (c) comprises an antenna located generallyat a line of demarcation between said first and second zones.
 29. Themethod claimed in claim 28 wherein said antenna is configured totransmit said second signals substantially in the same pattern in eachof said adjacent portions of said first and second zones.